The present invention is related to zeolite crystals, and more particularly, related to washing and recovering zeolite crystals, especially zeolite crystals less than about 0.5 microns in size.
Zeolite based catalysts are useful in catalytic reforming processes. Catalytic reforming is a major petroleum refining process used to raise the octane rating of naphthas (C.sub.6 to C.sub.11 hydrocarbons) for gasoline blending. Catalytic reforming is also a principal source of aromatic chemicals (benzene, toluene, and xylenes) via conversion of paraffins and naphthenes to aromatics. The principal chemical reactions which occur during catalytic reforming include dehydrogenation of cyclohexanes to aromatics, dehydrocyclization of paraffins to aromatics, dehydroisomerization of alkylcyclopentanes to aromatics, isomerization of normal paraffins to branched paraffins, dealkylation of alkylbenzenes and hydrocracking (or hydrogenolysis cracking) of paraffins to light hydrocarbons, i.e., methane, ethane, propane, and butane. The hydrocracking reaction is undesirable and should be minimized since it produces light hydrocarbons that are not suitable for gasoline blending and which are accordingly less valuable than fractions useable in gasoline.
Current reforming catalysts widely used in commercial reformers are platinum on an alumina substrate, and platinum plus a second promoting metal such as rhenium or iridium on alumina. These catalysts are bifunctional, i.e., the dehydrogenation reactions required in the reforming process are accomplished on the catalytic metal in the catalyst and the isomerization and cyclization reactions also required in reforming are accomplished on strong acid sites on the alumina catalyst support. Undesirable hydrocracking reactions which break down C.sub.6+ paraffins to lower molecular weight hydrocarbons and reduce selectivity to aromatics also occur on the strong acid catalytic sites.
Alumina based reforming catalysts demonstrate reasonably high selectivities for converting C.sub.8+ paraffins and naphthenes to aromatics but are less satisfactory for aromatizing C.sub.6 to C.sub.8 paraffins; they hydrocrack more of the lower paraffins to low value fuel gas than they convert to aromatics.
New reforming catalysts are being developed which are significantly more active and selective for aromatizing C.sub.6 to C.sub.8 paraffins than alumina based catalysts. These new catalysts are zeolite based rather than alumina based. Zeolite based reforming catalysts are more selective for aromatizing lower paraffins because they are monofunctional, i.e., they accomplish the isomerization and cyclization reactions on the same active catalytic metal sites on which the dehydrogenation reactions occur. To accomplish isomerization, they do not require nor contain strong acid sites which substantially eliminates hydrogenolysis cracking reactions.
In addition, certain zeolites employed to make such newer catalysts have micropore dimensions and physical configurations which sterically promote the desirable isomerization and dehydrocyclization reactions for C.sub.6 to C.sub.8 paraffins with adequate activity maintenance for commercial use and repress undesirable hydrogenolysis cracking reactions. Accordingly, selectivity of C.sub.6 to C.sub.8 paraffin conversion to aromatics is high for these sterically favored zeolite catalysts. Zeolites which perform best as reforming catalyst substrates fall into the so-called "large pore" category which have pore diameters of 6 angstrom units or higher. Large pore zeolites, such as zeolite L, are believed to be particularly good reforming catalyst substrates.
U.S. Pat. No. 4,448,891, to Cohen, teaches an improved reforming catalyst (employing a zeolite L support) made by continuously soaking the zeolite L (or using a series of soaks) in an alkali solution having a pH of at least 11 for a time and temperature effective to increase the period of time over which the catalytic activity of the catalyst is maintained, followed by conventional filtrate washing the alkali soaked zeolite with water (optionally followed by repeated soakings in the zeolite solution for additional periods with filtrate washing repeated after each soak) until the pH of the wash water coming off the zeolite filtrate is at or below 10.5, followed by drying at 110.degree. C.
U.S. Pat. Nos. 4,544,539 and 4,593,133, both to Wortel, teach zeolite L having desired characteristics achieved by a specified process. These Wortel patents teach a preferred form of zeolite L for use as a substrate for reforming catalysts. As noted in Example 15, the solid zeolite product was separated by centrifuging, washed four times with cold water and dried at 150.degree. C. for four hours. For this same example, the zeolite was in a cylindrical shape and 1.5 to 2.5 microns in size. For Example 17, the zeolite was in a cylindrical shape and 1 to 1.5 microns in size and for Example 1, 2 to 2.5 microns. The general teachings of these patents are for cylindrically shaped large crystallites with a mean diameter of at least 0.1 micron, with a mean diameter of at least 0.5 micron being preferred. Tables 3, 4, and 5 illustrate zeolite sizes for the patent's process and comparison processes; one comparison process (W of Table 3) provides zeolites of 0.1 to 0.2 micron in size.
U.S. Pat. No. 3,216,789, to Breck, teaches a process for producing synthetic zeolites which involves washing zeolite crystals, after the reactant mother liquor is filtered off, preferably with distilled water, until the effluent wash water, in equilibrium with the product, has a pH of between 9 and 12. The examples of this patent teach that the resulting zeolite crystals settled to the bottom of the crystallization jar leaving a clear supernatant of reactant mother liquor. This patent also discloses that as the zeolite crystals are washed, the exchangeable cation of the zeolite may be partially removed and is believed to be replaced by hydrogen cations. If the washing is discontinued when the pH of the effluent wash water is between about 10 and 11, then the (K.sub.2 O+Na.sub.2 O)A1.sub.2 O.sub.3 molar ratio of the crystalline product is disclosed as being approximately 1.0. The patent also notes that excessive washing will result in a somewhat lower value for this ratio, while insufficient washing will leave a slight excess of exchangeable cations associated with the product. The zeolite crystals are then dried, conveniently in a vented oven.
The conventional zeolites produced by the processes of Wortel (and probably Breck) are large particle zeolites having a length of about 0.9 to about 1.3 microns and a diameter of about 1.0 to about 1.3 microns. Newer small sized zeolites now being produced have a length of about 0.4 to about 0.7 micron and a diameter of about 0.3 to about 0.5 micron, although some particles may be smaller than these lengths and/or diameters. These newer small particle zeolites produce reforming catalysts believed to have a better activity, selectivity and activity maintenance than other known zeolite-based reforming catalysts. Examples of such small sized zeolite based reforming catalysts are disclosed in European patent application publication number 0 219 354 of Verduijn. However, it is often difficult to prepare such catalysts because of the difficulty of recovering and washing small zeolite particles.
Microfiltration (or ultrafiltration) is well known and has been employed in water purification and beverage processing where solids in a solution are considered a contaminant to be removed and are present in small concentrations of less than about 10 wt %. That is, generally the solute is the product and the solids are to be removed and discarded. For example, water desalination employs ultrafiltration to remove dissolved salt ions from water.
However, microfiltration (or ultrafiltration) may also be used to concentrate and wash solids in liquid streams from the order of 1 wt % to about 10 wt % where solids are the products. In biotechnology, microfiltration is used for concentration and recovery of proteins and bacteria from dilute solutions. Microfiltration is employed for particles in a size range below about 1.0 micron.
U.K. Patent Application 1,356,741 discloses a method for concentration and purification of particulate biological materials, having a particle size greater than about 50 m.mu., from growth medium components with a filter having pore sizes in the 0.22 to 0.65 .mu. range. In two examples, the biological particulate matter is concentrated with one pore size filter (0.45 .mu.) in a first thin-channel ultrafiltration module and then washed in a second thin-channel ultrafiltration module with a 0.22 .mu. pore filter. In the third example, the biological particulate was only concentrated in a thin-channel ultrafiltration module using a 0.65 .mu. pore filter. This patent teaches an initial concentration step employing one pore size ultrafilter followed by a washing step employing a different and smaller pore size ultrafilter.
U.S. Pat. No. 4,130,485 discloses a method for separating particulate solids (having a distribution in size of from about 0.1 to about 50 microns) from a solid/fluid dispersion (dye dispersion) via a solid, porous, tubular microfilter (such as sintered, stainless steel) with a pore size of between 0.5 to 5 microns. This patent teaches an initial washing step followed by a concentration (to about 11%, but no more than about 20% by weight) step. This patent employs a solid, 2 micron pore size, porous tubular member to wash and concentrate a mixture of a minor amount of primary particles and a predominant amount of flocculated particles of primary dye particles, with an average particle diameter in the slurry of about 3 microns and about 1% by weight dye particles. The volume of fluid is initially reduced by circulating slurry until a desired inlet pressure is obtained. Further an additional surfactant may be added to prevent any substantial pressure build-up during the concentration step.
The present invention employs microfiltration (or ultrafiltration) techniques to wash and recover zeolites crystals, especially zeolites less than about 0.5 microns in size, from crystalline mother liquor or other aqueous liquids to produce a superior catalyst substrate.